There may be two different mechanisms to refine the grains of casting alloy by adding refiners, and their effects on the properties of the alloy are quite different
(1) The reaction product precipitates out in large amount before the alloy begins to solidify and becomes heterogeneous nucleus, which makes the number of grains increase.
(2) The reaction products precipitated later, which can not be regarded as heterogeneous crystal nucleus, but only on the grain boundary. A large number of inclusions precipitated on the grain boundary can play a pinning effect, hinder grain growth, and refine the grain. However, inclusions on the grain boundary have a great negative effect on the mechanical properties of the alloy, especially on the plasticity and toughness. For example, adding Ti and Nb into the cast steel can refine the grain, but the refining mechanism is pinning, so the negative effect is great. Although a lot of research work has been done in this area, it is still useless.
In the solidification process of liquid metal, a large number of fine solid-phase crystal cores are formed in the liquid metal. As the core of the precipitated solid phase, each crystal core and the solid metal attached to it is a crystal. Then, the grains grow until all the molten metal solidifies. In the same amount of liquid metal, the more nuclei are formed, the finer the grains are.